Abstract. The thermo-mechanical fatigue (TMF) behaviour of the Nimonic 90 Nickel base superalloy has been investigated within two laboratories. In-phase-tests (IP) where the maximum mechanical strain occurs at the maximum temperature (850°C), and 180°-out-of-phase-tests (180° OP) where the maximum mechanical strain coincides with the minimum temperature (400°C) have been applied. All tests were carried out at varying mechanical strain ranges with a constant strain ratio of R ε = -1. A temperature rate of 5 K/s was used throughout the whole cycle without any additional cooling system during decreasing temperature. The fatigue life of 180° OP tests is longer compared to identical IP tests. The stress / mechanical strain hysteresis loops are completely different and some characteristic values are compared to each other. The fracture surfaces observed show that fatigue crack (or cracks) starts on the external surface and propagates inwards. The fractures of 180° OP tests are transgranular showing the presence of fatigue striations, while the fractures of IP tests are mixed transgranular and intergranular with no fatigue striations. IntroductionSince the seventies, materials subjected to cyclic stresses and temperatures have been studied by isothermal low cycle fatigue testing (LCF) and the results have been produced by considering a reference temperature that corresponds to the maximum value of the thermal cycling. After the introduction of thermo-mechanical fatigue (TMF) as a diagnostic device for the material study [1], the comparison of LCF and TMF testing has determined opposite conclusions. Partly LCF and TMF results were found to be comparable [2,3]. Partly it was found that the stress history of a thermomechanical cycle leads to results completely different from those produced by isothermal testing [4, 5,]. The TMF procedure is particularly recommended for those new materials whose high temperature properties are not known. There is clear evidence that the TMF test procedure does represent material loading of hot components in e.g. power stations and aero engines much better than LCF testing. In order to customise the TMF testing procedure a European project has recently concluded [6] and a Code of Practice (CoP) has produced [7]. The TMF tests were performed on the Nickel base superalloy Nimonic 90 at defined experimental conditions and the results contributed to the definition of the test procedure. The scope of the present work is to extend the comparison of TMF results performed on Ni90 alloy according to the CoP procedure and to verify the material life in different experimental conditions.
The nvo-way memory training across the 8 2 c) B19 transformation of a 50 al.% T i 4 al.% Ni-IO at.% Cu alloy has been invesiigaled here following both the strain and lhe electrical mistance change.Training loops of the form (P + M transformation + straia + shape recovery on ~v e x s e transformation) under applied stress levels of 51. 70.1, 89.2 and 108.3 MPa have been examined. The chief aim of the paper has been both to detect the electrical resislance change acmss the 8 2 CI 819 transformation, which is linearly related to the strain. and to discuss the different contributions related lo the electric transpofi pmpenies. The mulls found here provide sound support for selecting thc electrical resistance as a stare control variable in a lwo-way memory effect device.
It is well known that mechanical properties of Shape Memory Alloys are strongly dependent upon the test temperature (T) respect to transformation temperatures: the stress-strain curves however hinder the true deformation processes acting.Electrical resistance(ER), a physical property sensibly affected by electronic structure modifications, traditionally used to follow the growth of thermal martensite, is here investigated to follow the modifications of a NiTi alloy, in an initial single phase structure, under applied stress. ER measurements are here detected with the aim to distinguish different deformation processes at four test temperatures Ti (i=1,..,4): in martensitic phase,either at T1 <Mf or at T2<As within the hysteresis cycle; in parent phase, either at Af<T3 or at Ms<T4 within the hysteresis cycle, where T2 = T4. Results are examined in comparison with previous obtained data and discussed at the light of imprinted deformation.
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